This invention relates to improved reliability in devices which are connected to AC power when stationary and in use, but which must be powered by a rechargeable battery while away from AC power, and relates particularly to improved reliability in postage meter registers taken to a post office for resetting with additional postage.
A postage meter is used to print postage at a customer's premises away from the post office. A counter on the machine, called the descending register, records the quantity of postage that may be printed by one using the postage meter, and this counter is decremented each time a piece of mail is passed through the meter for printing of postage. It is imperative that only the post office and instrumentalities under its control be allowed to add to the descending register, otherwise one could give oneself free postage without paying the post office. It would be much too heavy and awkward to carry the entirety of the meter to the post office for each resetting operation, called a resetting, so the postage meter is usually designed so as to be divided into a fixed part (the "base") meant to stay in the customer's premises and a portable part (the "meter portion") that includes the descending register. Thus, only the meter portion need be carried to the post office for resetting, which is done by a postal service employee after the customer has paid the postal service for the postage to be added to the descending register.
It is known to use mechanical means for the descending register. The disadvantage of such means is apparent when one is forced to carry such a meter portion for a resetting at the local post office--mechanical means are bulky and heavy. Also the resetting must be effectuated mechanically, usually by turning a crank, a cumbersome and time-consuming process.
It is preferable to use lighter and more compact electronic means for storage of the descending register, and this requires a reliable portable power supply to power the electronic means of the meter portion during resetting. Generally, two power supplies are needed. One, a very small power supply, maintains the contents of a static RAM memory with high reliability and great longevity. A larger power supply is also needed during the resetting session to power the remainder of the electronics, which include a processor, a display, a keyboard, and other related circuitry. This larger power supply is also relied upon for continuous powering of certain circuit elements (such as a clock/calendar circuit) the continued operation of which is desirable but less crucial than maintaining the descending register value. The larger power supply is typically a rechargeable battery such as a nickel-cadmium (or lead-acid) sealed battery.
The base contains a power supply in addition to those mentioned above. During times when the meter is connected to AC power (i.e. through the base) and powered up, the base power supply powers the electronics, recharges the rechargeable batteries, and provides all other power required for any and all meter functions.
Two factors, then, contribute to the possibility of exhaustion of the larger power supply. First, if the meter is allowed to sit for many days or weeks without being connected to the AC power supply, the small but non-negligible drain of the clock/calendar will have drained the rechargeable batteries to less than full charge. Second, if the rechargeable batteries have been discharged for whatever reason (such as activation of the electronics during a post office trip) then the amount of time the meter portion has been recharging (presumably because it is back on the base after the completion of the trip) may not yet have been enough to recharge the rechargeable batteries fully.
The very small power supply is preferably located as part of a sealed unit with its associated static RAM memory, and for the purposes of this discussion is assumed to be reliable even in the face of loss of power to any and all other parts of the meter. Where the memory device is a CMOS memory and the power supply is a lithium cell, the life is assumed to be on the order of years, because the quiescent power drain of the memory is on the order of microamperes.
The larger power supply, however, lasts not years but minutes, because the power consumed when the electronics are in use (e.g. at the post office) is on the order of tens or hundreds of milliamperes. The power supply life is limited by the fact that physical size of the rechargeable battery is constrained and ratio of capacity to size is smaller for rechargeable batteries such as the preferred nickel-cadmium (nicad) batteries than for nonrechargeable batteries.
If the electronics power (from the larger power supply) fails while the meter portion is in transit or during the resetting session, the trip to the post office must be made again after recharging. Although nonrechargeable batteries satisfy the portability requirement, they are expensive and require replacement. To maintain a safety factor, it might be necessary to buy new batteries for each trip lest normal battery deterioration over time give rise to the above-mentioned problems.
There is another reason why it is desirable that the larger power supply not fail, whether in transit or at other times. The above-mentioned small power supply is backed up by the large one, so that if the lithium cell happens to run down, the rechargeable batteries will nonetheless preserve the contents of the CMOS memory, thereby protecting the crucial descending register information.